1. Field of the Invention
The present invention relates to a method of inspecting stray light that occurs in a radiation image reader in which excitation light is illuminated to a storable fluorescent sheet having stored and recorded a radiation image, to obtain an image signal representing the radiation image, from the storable fluorescent sheet.
2. Description of the Related Art
Radiation recording-reproducing systems have been proposed in Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-164645, 55(1980)-116340, etc. These systems utilize a storable phosphor (stimulatable phosphor), which stores part of radiation energy when illuminated with radiation (such as X-rays, α-rays, β-rays, γ-rays, electron rays, ultraviolet rays, etc.) and emits photostimulated luminescent light in accordance with the quantity of the stored radiation energy when illuminated with excitation light (such as visible light, etc.). The radiation image of a subject, such as a human body, etc., is temporarily photographed and recorded on a storable fluorescent sheet. The storable fluorescent sheet with the radiation image emits photostimulated luminescent light when scanned with excitation light such as a light beam, etc. The photostimulated luminescent light obtained is photoelectrically read and converted to an image signal by photoelectric reading means such as a photomultiplier, etc. Based on the image signal, the radiation image of the subject is output as a visible image to a recording material such as photosensitive material, etc., or to a cathode-ray tube (CRT) display unit, etc.
A radiation image reader used in the aforementioned radiation recording-reproducing system is equipped with a light source for emitting a light beam (excitation light), a rotating polygon mirror (deflection means) for reflecting and deflecting the light beam emitted from the light source, and a scanning optics system, which consists of an fθ lens, a cylindrical lens, and a cylindrical mirror. The light beam, reflected and deflected by the deflection means, is reflected by the cylindrical mirror to change the optical path and is scanned on the scan surface of the storable fluorescent sheet. Thus, the optical path of the light beam can be assured without increasing the size of the reader, by reflecting the light beam with the cylindrical mirror to change the optical path.
In the above-mentioned scanning optics system, however, the light beam is reflected at the scan surface of the storable fluorescent sheet, and the reflected light beam is further reflected by the cylindrical mirror and returns to the cylindrical lens. Because of this, there is a possibility that the return light will be reflected at the surface of the cylindrical lens and will be incident again upon the scan surface as stray light. In certain cases, the light beam, reflected and deflected by the deflection means, is reflected by the scanning optics means and returns to the deflection means, and this return light is incident again on the scan surface as stray light. The stray light always occurs at the same position on the horizontal scanning line. Therefore, if stray light occurs in the radiation image reader mentioned above, the storable fluorescent sheet will be excited by this stray light and emit photostimulated fluorescent light, and consequently, noise in the form of lines extending in the vertical scanning direction will develop in the obtained image signal. Therefore, it is necessary to inspect stray light when shipping radiation image readers and to ship only readers having passed the inspection.